cpufreq: Convert the cpufreq_driver_lock to use RCU

In general rwlocks are discourged so we are moving it to use RCU instead.

Signed-off-by: Nathan Zimmer <nzimmer@sgi.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
modified for Mako from kernel.org

Signed-off-by: faux123 <reioux@gmail.com>